Osteosynthetic pin

ABSTRACT

An osteosynthetic pin characterized in that the pin is formed substantially of a poly-L-lactic acid having a molecular weight of at least about 70,000, and is formed by axially drawing about 2 to about 10 times at an elevated temperature of about 70° to about 120° C.

TECHNICAL FIELD

The present invention relates to bone fixing pins for use in surgery,especially for use with cut ribs, fractured bones or the like for fixingand remedying the bones.

BACKGROUND ART

Metal or alumina ceramic osteosynthetic pins have heretofore been usedclinically for remedying cut or fractured bones by inserting the pininto the marrow cavity of the cut or fractured bone and fixing the bone.

However, these osteosynthetic pins have the disadvantages of being lowin elasticity, exerting no shock-absorbing action relative to themovement of the fixed bone portion and stimulating the bone to cause aninflammation or retard osteogenesis. Moreover, they have poor affinityfor the living body, are not absorbable by the living body and thereforehave the problem of permanently remaining in the living body after thefracture has been remedied, possibly causing contamination with bacteriaor an inflammation.

DISCLOSURE OF THE INVENTION

An object of the present invention is to provide an osteosynthetic pinhaving suitable elasticity and high affinity for the living body,absorbable by the living body after surgery and advantageously usable inregenerating and remedying the bone without causing an inflammation tothe fixed portion and further without entailing the objection that couldresult if the pin remains permanently in the living body.

Another object of the invention is to provide an osteosynthetic pincapable of exhibiting an osteogenesis promoting effect on fixed bones.

The osteosynthetic pin of the present invention is characterized in thatthe pin is formed substantially of a poly-L-lactic acid having amolecular weight of at least about 70,000.

We have conducted research on various high-molecular-weight substanceshaving affinity for living body and absorbable by the living body toexplore whether these substances are usable for osteosynthetic pins.Many of the high-molecular-weight substances investigated did not havesuitable elasticity required of osteosynthetic pins, were low in initialstrength or failed to retain the required strength during the boneregeneration period (usually about 2 months), and were thereforeunsuited to use for osteosynthetic pins. Nevertheless, we have foundthat of the homopolymers of L-lactic acid, only poly-L-lactic acidhaving a molecular weight of at least 70,000 is excellent in initialstrength, retains the required strength during the bone regenerationperiod, permits effective regeneration and remedy of bones and istherefore very suitable for osteosynthetic pins. Moreover, thepoly-L-lactic acid posseses suitable elasticity, accordingly exhibits ashock-absorbing action relative to the movement of the fixed boneportion and is unlikely to cause an inflammation or retard regenerationof bones by stimulating bones. The poly-L-lactic acid further hasexcellent affinity for the intramedullary tissue when placed into themarrow cavity and is gradually absorbed by the living body throughhydrolysis therein, so that the acid is usable free of the objectionthat would result if the acid remains permanently in the living bodylike metal or alumina ceramics.

Although the poly-L-lactic acid to be used in the present invention isitself known, no report has been made on the acid as used forosteosynthetic pins. Homopolymers of L-lactic acid which are less thanabout 70,000 in molecular weight are not satisfactory in the propertiesrequired of osteosynthetic pins, i.e. in initial strength and strengthretentivity during the bone regeneration period, and are not usable forthe pin. The poly-L-lactic acid suited to the present invention is atleast about 70,000, more preferably at least about 100,000 and mostpreferably at least 150,000, in molecular weight. The upper limit of themolecular weight, although not limited specifically, is generally about150,000 in view of shapability, etc.

The molecular weight of the poly-L-lactic acid mentioned herein isdetermined by dissolving the acid in chloroform to a concentration of0.2 g/d1, further diluting the solution to determine the intrisincviscosity [η], and calculating the viscosity average molecular weightfrom the following equation. ##EQU1##

To give the osteosynthetic pin increased affinity to the living body andenable the pin to exhibit a bone regeneration promoting effect whenfixing bones, it is desirable to use the poly-L-lactic acid withhydroxyapatite (hereinafter referred to as "HAP") admixed therewith. HAPis a known substance in the form of inorganic crystals represented bythe formula Ca₁₀ (PO₄)₆ (OH)₂. According to the invention, HAP is usedin the form of particles, preferably about 1 μm to about 100 μm indiameter, as uniformly admixed with the poly-L-lactic acid, usually inan amount of about 1 to about 15 wt. %, preferably about 5 to about 10wt. %, based on the poly-L-lactic acid. HAP can be admixed with thelactic acid by any desired method, for example, by mixing the twomaterials together in the form of powders, or by uniformly mixing HAPwith a solution of the lactic acid polymer and evaporating off thesolvent from the mixture. Further to assure promoted regeneration of thebone and improved strength, calcium phosphate fiber obtained from HAPcan be admixed with the poly-L-lactic acid for preparing the pin of theinvention.

The present osteosynthetic pin can be of any desired shape, for example,in the form of a plate, polygonal prism (such as quadrangular,pentagonal, hexagonal or octagonal prism), solid cylinder or the like.For example, the pin can be in the form of a plate or polygonal prismwhich is about 2 to about 7 mm in thickness, about 2 to about 7 mm inwidth and about 20 to about 50 mm in length, or a solid cylinder about 3to about 7 mm in diameter and about 20 to about 50 mm in length.

With reference to the accompanying drawings showing preferred examplesof osteosynthetic pins embodying the invention,

FIG. 1 is a perspective view showing an osteosynthetic pin in the formof a plate, and

FIG. 2 is a perspective view showing an osteosynthetic pin in the formof a solid cylinder.

Referring to these drawings, the pin 1 of the invention has taperedopposite ends 2, which are desirable for facilitating the insertion ofthe pin into the marrow cavity of the bone. Indicated at 3 in FIG. 1 isa bore which extends through the pin axially thereof and which is formedwhen desired. When provided, the bore achieves an enhanced curing effectwithout impeding the growth of myeloid cells.

The osteosynthetic pin of the present invention is prepared by shapingthe poly-L-lactic acid, or a mixture of the acid and HAP. Thepoly-L-lactic acid has excellent shapability at a relatively lowtemperature and therefore readily affords osteosynthetic pins in adesired shape. The pin is produced, for example, by shaping the materialat an elevated temperature of not lower than the melting point of thepoly-L-lactic acid using a hot press, or by extruding the material froma nozzle of suitable orifice diameter at a temperature of not lower thanthe melting point, drawing the extrudate axially thereof and, whenrequired, tapering the resulting piece. The axial drawing gives improvedelasticity and strength to the pin. The extrudate is drawn at anelevated temperature usually of about 70° to about 120° C., preferablyabout 2 to about 10 times axially.

The osteosynthetic pin of the invention is inserted into the marrowcavity of a cut or fractured bone to fix and remedy the bone. The pin isadvantageously usable for remedying fractured or out ribs and also forfixing and remedying fractured or cut bones in various body portions.

The features of the invention will be made apparent from the followingexamples of the invention.

EXAMPLES Example 1

A powder of poly-L-lactic acid, about 123,000 in molecular weight, washeld in a hot press at 200° C. for 5 minutes free of any pressure tomelt the lactic acid, and the melt was then held at the same pressurefor 5 minutes under a pressure of 200 kg/cm² and thereafter cooledrapidly to obtain a 3-mm-thick plate of poly-L-lactic acid. Rib fixingpins having the shape shown in FIG. 1, a width of 6 mm and a length of40 mm were prepared from the plate by cutting.

The rib fixing pins were immersed in a phosphate buffer (PBS) containing0:9 wt. % of sodium chloride at 37° C. for a hydrolysis test todetermine the relationship between the immersion period and weight,bending strength or bending Young's modulus. Table 1 shows the results.

                  TABLE 1                                                         ______________________________________                                                 Immersion period                                                                         1       2     3     6                                     Item       Initial  Month   Months                                                                              Months                                                                              Months                                ______________________________________                                        Weight     100      100     100   100   100                                   retentivity                                                                   (%)                                                                           Bending    11.1     11.2    12.6  11.4  3.7                                   strength                                                                      (kg/mm.sup.2)                                                                 Bending strength                                                                         100      101     114   102   33                                    retentivity                                                                   (%)                                                                           Bending Young's                                                                          288      296     319   314   289                                   modulus                                                                       (kg/mm.sup.2)                                                                 Bending Young's                                                                          100      103     111   109   100                                   modulus                                                                       retentivity                                                                   (%)                                                                           ______________________________________                                    

The bending strength and bending Young's modulus were measured accordingto JIS K7203. The weight retentivity, bending strength retentivity andbending Young's modulus retentivity are expressed in percentage relativeto the respective initial values.

Example 2

Poly-L-lactic acid, about 72,000 in molecular weight, was dissolved inmethylene chloride, 5 wt. % of hydroxyapatite (proportions by weight ofmain components: 55.8% CaO, 42.3% P₂ O₅ and 0.3% MgO; particle size upto 0.088 mm) was added to the solution, the mixture was stirred and thesolvent was removed from the mixture to obtain a homogeneous materialcontaining hydroxyapatite. The material was shaped into a plate by a hotpress, and the plate was then drawn two times in hot water at 80° C. andmade into osteosynthetic pins each in the form of a solid cylinder, 2 mmin diameter and 20 mm in length.

The pins were 16.2 kg/mm² in initial bending strength and retained 80%of the initial strength after having been tested in vitro as immersed inPBS for 2 months.

Seven hybrid adult dogs (weighing 7 to 15 kg) were intramuscularyanesthetized with 10 mg/kg of Ketaral and 0.1 mg/kg of Rompun andsubject to mechanical ventilation under positive pressure whileintubated with a tracheal tube to perform the following surgery usingsome of the pins of the present example and osteosynthetic stainlesssteel pins for comparison.

The left 9th, 10 and 11th ribs were cut inclusive of the periosteum at asite about 2 to about 4 cm from the costal angle, and the cylindricalpin was inserted into the marrow cavity of each rib to join the cut ribat the site. The rib was tied with #7 silk ligature at a proximal siteand a distal site each at a distance of 1.5 cm from the cut end, and thetied pieces of ligature were tied to each other to fix the joinedportion. The fixed portion was thereafter checked for a cure from timeto time by X-ray examination, and the fixed portion was removed en bloc2 weeks, 3 weeks, 1 month, 2 months or 3 months after the surgery tocheck the rib for tissue reaction and also check the material for achange.

Consequently, the osteosynthetic pin of poly-L-lactic acid was found toexhibit high affinity for the living body, and the bone was found tohave been completely fixed one month after the surgery. On the otherhand, the stainless steel pin exhibited poor affinity for theintramedullary tissues and was found to have been surrounded by aconnective tissue layer. At the portion where the pin was in directcontact with the bone, the bone became thin, permitting the pin to slipoff in some cases.

Example 3

To 100 parts by weight of a powder of poly-L-lactic acid, about 72,000in molecular weight, was added 5 parts by weight of the samehydroxyapatite as used in Example 2, and the mixture was thoroughlystirred and then extruded into a rod from a nozzle having a singleorifice with a diameter of 3 mm at a temperature of 230° C. using asingle-screw spinning machine. The rod of poly-L-lactic acid containinghydroxyapatite was axially drawn two times in air at about 80° C. andthereafter cut into pieces having a length of 40 mm. Each of the pieceswas tapered at its opposite ends to obtain a rib fixing pin as shown inFIG. 2.

The osteosynthetic pins thus prepared had the strength shown in Table 2below, which also shows the strength of the rod before the drawing.

                  TABLE 2                                                         ______________________________________                                                       Before After                                                                  drawing                                                                              drawing                                                 ______________________________________                                        Bending strength 11.2     16.2                                                (kg/mm.sup.2)                                                                 Bending Young's  184      462                                                 modulus                                                                       (kg/mm.sup.2)                                                                 ______________________________________                                    

Some of the rib fixing pins were subjected to a hydrolysis test(hereinafter referred to as an "in vitro test") in a phosphate buffer(PBS) containing 0.9 wt. % of sodium chloride. Table 3 shows theresults.

                  TABLE 3                                                         ______________________________________                                                 Immersion period                                                                       Months                                                      Item       Initial 2 Weeks  1    2    3    6                                  ______________________________________                                        Weight                                                                        retentivity                                                                              100     100      100  100  100  98                                 (%)                                                                           Bending                                                                       strength   16.2    13.7     12.9 12.8 10.0 3.8                                (kg/mm.sup.2)                                                                 Bending strength                                                              retentivity                                                                              100     85       80   79   62   24                                 (%)                                                                           Bending Young's                                                               modulus    462     383      445  421  376  262                                (kg/mm.sup.2)                                                                 Bending Young's                                                               modulus    100     83       96   91   81   57                                 retentivity                                                                   (%)                                                                           ______________________________________                                    

Next, cut ribs of adult dogs were fixed each with the pin and checkedfor fixation. Although the cut portion was not yet completely fixed twoweeks thereafter, the rib fixing pin was found to have been firmlyjoined to the intramedullary tissue, thus satisfactory serving as anosteosynthetic pin.

For comparison, 2-cm-long cut pieces of Kirschner's wire, 2 mm indiameter, were tested in vivo in the same manner as above. Two weeksafter the start of testing, the Kirschner's wire was found to be looselypositioned in the bone without joining to the intramedullary tissue inany way and was readily movable within the bone marrow when pressed onfrom outside the living body, thus stimulating the tissue and failing toserve as a fixing pin.

We claim:
 1. An osteosynthetic pin obtained by shaping a poly-L-lacticacid having a molecular weight of at least about 70,000 and axiallydrawing the shaped body about 2 to about 10 times at an elevatedtemperature of about 70° to about 120° C.
 2. An osteosynthetic pin asdefined in claim 1 wherein the poly-L-lactic acid has a molecular weightof at least about 100,000.
 3. An osteosynthetic pin as defined in claim1 wherein the poly-L-lactic acid contains about 1 to about 15 wt % ofhydroxyapatite admixed with the poly-L-lactic acid.
 4. An osteosyntheticpin as defined in claim 3 wherein the content of hydroxyapatite is inthe range of about 5 to about 10 wt %.
 5. An osteosynthetic pin asdefined in claim 1 or 3 further comprising a bore extending axiallythrough the pin.
 6. A process for preparing an osteosynthetic pin,omprising: shaping a poly-L-lactic acid having a molecular weight of atleast about 70,000 and axially drawing the shaped body about 2 to about10 times at an elevated temperature of about 70° to about 20° C.